Until 2000, the authorized sulfur content in diesel fuel was 350 ppm. Much more stringent values have been imposed since 2005 since this maximum content is not to exceed 50 ppm. This maximum value will next be revised downward and should not exceed 10 ppm in a few years.
It is thus necessary to develop processes meeting these requirements without prohibitively increasing the cost of production.
Gasolines and gas oils resulting from the conversion process, such as, for example, hydroconversion, are very refractory in hydrotreatment compared to gas oils that are obtained directly from the atmospheric distillation of crude oils.
To obtain very low sulfur contents, it is necessary to convert the most refractory types, especially di- and trialkylated dibenzothiophenes, or those having a greater degree of alkylation, for which access of the sulfur atom to the catalyst is limited by the alkyl groups. For this family of compounds, the route of hydrogenation of an aromatic cycle before the desulfurization by breaking the Csp3-S bond is faster than direct desulfurization by breaking the Csp2-S bond.
It is likewise necessary to obtain a major reduction of nitrogen content by conversion especially of the most refractory types, especially benzacridines and benzocarbazoles; the acridines are not only refractory, but also inhibit hydrogenation reactions.
Conversion gas oils thus require very rigorous operating conditions to obtain the desired sulfur specifications.
A process of conversion of heavy petroleum fractions including a boiling bed for producing middle distillates with a low sulfur content has been described especially in Patent Application EP 1 312 661. This process, however, makes it possible to reduce sulfur levels below 50 ppm only under very rigorous pressure conditions, which greatly increases the cost of the gas oil that is ultimately obtained.
There is thus a genuine need for a process making it possible to hydrotreat conversion gas oils under less rigorous operating conditions allowing a reduction in investment costs while maintaining a reasonable cycle duration of the hydrotreatment catalyst and allowing sulfur contents of less than 50 ppm, preferably less than 20 ppm, and more preferably less than 10 ppm, to be obtained.
Values in ppm are all expressed by weight.